1. Field of the Invention
The present invention relates to a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a method for driving these devices, and a method for manufacturing these devices. The present invention particularly relates to a semiconductor device, a display device, a liquid crystal display device, a light-emitting device which include driver circuits formed over the same substrates as pixel portions, and a method for driving these devices. Alternatively, the present invention relates to an electronic device including the semiconductor device, the display device, the liquid crystal display device, or the light-emitting device.
2. Description of the Related Art
In recent years, with the increase of large display devices such as liquid crystal televisions, display devices have been actively developed. In particular, a technique for forming a driver circuit such as a gate driver over the substrate, over which a pixel portion is provided, by using transistors including a non-single-crystal semiconductor has been actively developed because the technique greatly contributes to reduction in cost and improvement in reliability.
In a transistor including a non-single-crystal semiconductor, degradation such as variation in threshold voltage or reduction in mobility occurs. As such degradation of a transistor progresses, it becomes difficult to operate a driver circuit and incapable of displaying images. Patent Documents 1 and 2, and Non-patent Document 1 each disclose a shift register in which degradation of transistors that have a function of maintaining an output signal from a flip-flop at a low level or making an output signal from a flip-flop the low-level (hereinafter such a transistor is also referred to as a pull-down transistor) can be suppressed. In these documents, two pull-down transistors are used. These two pull-down transistors are connected between an output terminal of a flip-flop and a wiring to which VSS (also referred to as negative power supply) is supplied. Moreover, one pull-down transistor and the other pull-down transistor are alternately turned on (i.e., it can also be said that one pull-down transistor and the other pull-down transistor alternately go into an on state). Accordingly, the time during which each of the pull-down transistors is on is reduced, so that degradation of characteristics of the pull-down transistors can be suppressed.